Various types of hydraulic continuously variable transmissions have been proposed in constructions comprising a hydraulic pump and a hydraulic motor, at least one of which is of a variable displacement type. Also, plunger type swash plate pumps and motors are well-known as a pump and a motor which can be used in variable displacement for such transmissions. In addition, there is another type of hydraulic continuously variable transmission which is known as hydromechanical continuously variable transmission (e.g., Japanese Laid-Open Patent Publication No. S62(1987)-147148). This type of transmission includes a mechanism for carrying out mechanical power transmission, which is provided parallel to a power transmission mechanism that comprises a hydraulic pump and a hydraulic motor. The above mentioned former type, which comprises only a hydraulic pump and a hydraulic motor, is generally referred to as "hydrostatic continuously variable transmission". Therefore, here, both types, including the latter, are termed as "hydraulic continuously variable transmission."
Such hydraulic continuously variable transmissions require simultaneous control of the throttling of the engine and the varying of the capacities of the hydraulic pump and the hydraulic motor. For this reason, a number of control methods are practiced at present, and, for example, Japanese Patent Publication No. 2566457 discloses a method for controlling the speed change ratio (i.e., a method for controlling the capacities of the pump and the motor). In this method, the actual engine rotational speed is brought to a target engine rotational speed that is determined in relation to the degree of operation of the accelerator (i.e., the engine throttle opening).
In the vehicle which is controlled by this control method, when the brake pedal is stepped down, generally the accelerator pedal is released. This return of the accelerator pedal causes the accelerator to come into a complete closure, to which condition, a small target engine rotational speed is assigned. Generally, when the target engine rotational speed becomes small, the speed change is controlled toward the TOP. For example, if the speed of the vehicle decreases gradually in response to a normal operation of the brake, then the speed change ratio is adjusted toward the TOP (or it stays at the TOP if it was there before the operation of the brake). Thereafter, the engine rotational speed decreases as the speed of the vehicle decreases, and when it reaches the target rotational speed or below, the speed change ratio is then controlled to shift toward the LOW. Along with this control, in the transmission that is described in the above mentioned patent publication, when the speed of the vehicle decreases to a certain speed, another control is carried out to open a clutch valve that shortcircuits the hydraulic closed circuit which is provided for the operation of the hydraulic pump and the hydraulic motor.
However, if the wheels of the vehicle are locked up in an abrupt braking operation, then the rotational speed of the output shaft of the hydraulic motor decreases to zero in response to the lock up of the wheels, and the rotational speed of the hydraulic pump also decreases to zero very rapidly. As a result, the rotational speed of the engine, which drives the pump, also deceases rapidly, and this condition may cause a problem of engine stall.
In the transmission which is described in the above Patent Publication No. 2566457, for preventing the engine from stalling, the target engine rotational speed is amended to a larger value when the brake is operated, so that the speed change ratio shifts toward the LOW earlier, and the engine rotational speed is prevented from decreasing too much to cause an engine stall. However, generally the response of the shift control of the speed change ratio is low, so the decrease of the engine rotational speed which results from the wheel lock caused by a quick or abrupt application of the brake occurs before the start of the control to shift the speed change ratio toward the LOW, and the problem of engine stall is likely to happen.
Therefore, the transmission of the above patent publication includes a main clutch valve that shortcircuits the two oil passages which constitute the hydraulic closed circuit, and when the speed of the vehicle decreases, this main clutch valve is opened to prevent the engine from stalling. However, most hydraulic continuously variable transmissions are not equipped with this kind of valve, and they are designed to establish a neutral condition by bringing the skew angle of the swash plate of the pump to zero degree. However, when such an abrupt or quick braking operation is applied as to lock up the wheels, the response of the skew angle control of the swash plate fails to establish the neutral condition. In this condition, there may be a problem that the engine rotational speed may decrease rapidly, thus causing an engine stall.